1. Field of the Invention
The present invention concerns a method of MR imaging (MR, magnetic resonance).
2. Description of the Prior Art
In MR imaging based on the phenomenon of nuclear magnetic resonance, the test object is exposed to a static homogeneous magnetic field, whereby the atom's nuclear spins, which at first are randomly oriented, align along the axis of the static magnetic field. The ordered nuclear spins are excited by magnetic high frequency fields. For image formation, the nuclear spin signal is recorded as a voltage which is induced into one or several coils under the influence of a suitable sequence or progression of high frequency and gradient pulses in the time domain. Finally, the image reconstruction in general is carried out by means of a Fourier transform of the time signals. The number, the time interval, the duration and the strength of the gradient pulses employed define the mode of sampling of the reciprocal so-called “k-space”, which in turn determines the FOV (field of view) to be imaged as well as the image resolution.
In the advancement of the art of MR imaging methods, numerous efforts have already been made to shorten the image acquisition time and/or to improve the achievable spatial resolution.
To accelerate the imaging process, parallel MR techniques for example take advantage of the phenomenon that the spatial sensitivity profile of the receiver coils provides the nuclear spin signal with spatial information, which can be used for the image reconstruction. By using in parallel several separate receiver coils, each with a different sensitivity profile, and combining the respective nuclear spin signals detected, the acquisition time for an image can be reduced in comparison to a conventional reconstruction by a factor which is limited by the number of receiver coils and their spatial arrangement.
Further, for MR images where the side lobes of the SRF (spatial response function) entail severe contamination, it has been proposed to use a k-space filter which enhances the data in the centre of the k-space. While this filtering reduces the side lobes of the SRS and thereby the contamination from neighbor voxels, it also affects the spatial resolution and reduces the signal to noise-ratio (SNR). One approach to overcome this problem involves the use of density weighted or acquisition weighted imaging methods. This way, k-space filtering is already applied during data acquisition. If acquisition weighted methods are employed, the Nyquist criterion is generally met in the entire k-space. However, due to the greater number of acquisitions in the centre of the k-space and the need for a greater number of phase encoding steps for the same size of the FOV and the same spatial resolution as in an unweighted method, the minimum number of scans is restricted and thereby limited to the shortest recording duration. To overcome this problem, Greiser and von Kienlin have introduced the density weighted phase encoding method, which is described in more detail in the article “Greiser A, von Kienlin M., Efficient k-space Sampling by Density-Weighted Phase-Encoding. Magn Reson Med 2003; 50:1266-1275”. In contrast to the acquisition weighted methods, this method is based on the variation of the density of the sampling points in the k-space. The acquisition weighted sampling is a density weighted sampling in which specifically the k-space density is varied by a variable number of acquisitions on a fixed k-space grid. The density weighted sampling has proven to be a highly efficient method of k-space sapling. The modification of the sampling scheme makes it possible to improve the shape of the SRF and thereby the localisation properties. In the kind of density weighted sampling presently known, some parts of the k-space violate the Nyquist criterion, which entails undersampling artefacts.